Designer Jill Stuart’s husband and partner Ron Curtis has been slammed by a Supreme Court judge who accused Curtis of trying to intimidate a witness in a case against the duo.

Companies owned by Curtis and Stuart are being sued for non-payment of a $2.9 million bill. Curtis was accused of sending a letter to witnesses in the case. The court “regards the above letter as a serious attack upon the integrity of this lawsuit,” said Judge Herman Cahn, who enjoined Curtis from contacting witnesses.

The suit was filed by Vicki Ross and Richard Kramer, who claim they are owed the $2.9 million for establishing Jill Stuart’s licensed business with Itochu Fashion Systems Co. Ltd. in Japan.

Ross and Kramer charge that the defendants created Jill Stuart Japan to evade paying royalties due to them from JS Licensing for arranging their relationship with Itochu initially. The case is scheduled to start in Manhattan Supreme Court on May 21.

The plaintiffs alleged that Curtis attempted to sway the testimony of an employee of Itochu Fashion, a witness for the plaintiff, by writing a letter to her superior suggesting that all company personnel should “read the [licensing] agreement before giving testimony.”

The court interpreted Curtis’ letter as interference in its proceedings.

“The letter to Itochu coming when it did in the middle of the testimony of an Itochu witness clearly seems to be a bold attempt to intimidate the witness,” the judge wrote.

The Itochu employee, Ai Hirakawa, testified that the 1996 agreement between Itochu and JS Licensing had been renewed and there was no difference between JS Licensing and Jill Stuart Japan.

Jill Stuart’s business is enormous in Japan. Curtis, also president and CEO of Jill Stuart, has said sales in Japan are estimated at $100 million.

“Her sales in Japan are probably better than in the U.S. or anywhere else,” said Donald Kreindler, lawyer for Ross and Kramer. “. . . the first payment was made. Then it came time to renew and Ron Curtis created a new arrangement with four companies he was the sole owner of.”

Ron Curtis also is president of JS International, JS Retailing and JSNY, and the manager of JS Licensing and Jill Stuart Japan.

“The new [licensing] agreement said that JS Licensing Japan would pay 118 percent of moneys received to Jill Stuart entities. This was a sure way to ensure the company that owed money to Ross and Kramer would always be insolvent.”

Ron Curtis declined to comment. Ross and Kramer also declined to comment.